Fifteen intact female monkeys were placed into three groups of five, receiving either an 18, 32 or 16 mg capsule of Levonorgestrel (LNG) placed subdermally in the inter-scapular region on each animal, in groups I through III respectively. One pre-treatment blood sample was obtained, with post treatment blood collections daily from 1 through 7, once per week for 9 months for groups I and II and 6 months for group III, then biweekly for all groups until levels of LNG were undetectable. During this year, only animals from group I had blood samples collected for LNG analysis. Serum LNG was measured by radio-immunoassay. All animals from groups II and III were not monitored during this year because serum LNG was no longer detectable in these animals. LNG levels from group I continued to drop from 0.36 ng/ml (SEM 0.21) on day 1155 to 0.08 ng/ml (SEM 0.06) on day 1365, when all but one animal dropped to undetectable levels. The remaining animal continued to exhibit levels of LNG at 0.32 ng/ml on day 1379, then gradually dropped to undetectable by day 1509. *KEY*Levonorgestrel, Steroidal contraception, Subdermal implants, Synthetic progestin